Intermittent dieting achieved fad diet status in the UK when the Fast Diet book came out and a related TV programme.

I read the Fast Diet book.

It was interesting, but I think the evidence is inconclusive in terms of intermittent fasting vs small meals/less snacks approach. Likewise in terms of shorter vs longer fasts.

I was going to try intermittent fasting, but when I when to a vegan diet I found I no longer needed to consider this. I do try to eat 1 piece of fruit for breakfast at least once a week (i.e. a small breakfast that leaves me hungry), or a small salad for lunch once a week if my weight is say 2 kilos above my ideal, but that's about it.

I personally faint and feel low energy when I do eat hardly any food for a day or two. I would take things slowly to start with if you have any concerns about driving a car or have other things where alertness could be a factor, e.g. operating dangerous machinery at work.

Humans in modern societies typically consume food at least three times daily, while laboratory animals are fed ad libitum. Overconsumption of food with such eating patterns often leads to metabolic morbidities (insulin resistance, excessive accumulation of visceral fat, etc.), particularly when associated with a sedentary lifestyle. Because animals, including humans, evolved in environments where food was relatively scarce, they developed numerous adaptations that enabled them to function at a high level, both physically and cognitively, when in a food-deprived/fasted state. Intermittent fasting (IF) encompasses eating patterns in which individuals go extended time periods (e.g., 16-48h) with little or no energy intake, with intervening periods of normal food intake, on a recurring basis. We use the term periodic fasting (PF) to refer to IF with periods of fasting or fasting mimicking diets lasting from 2 to as many as 21 or more days. In laboratory rats and mice IF and PF have profound beneficial effects on many different indices of health and, importantly, can counteract disease processes and improve functional outcome in experimental models of a wide range of age-related disorders including diabetes, cardiovascular disease, cancers and neurological disorders such as Alzheimer's disease Parkinson's disease and stroke. Studies of IF (e.g., 60% energy restriction on 2days per week or every other day), PF (e.g., a 5day diet providing 750-1100kcal) and time-restricted feeding (TRF; limiting the daily period of food intake to 8h or less) in normal and overweight human subjects have demonstrated efficacy for weight loss and improvements in multiple health indicators including insulin resistance and reductions in risk factors for cardiovascular disease. The cellular and molecular mechanisms by which IF improves health and counteracts disease processes involve activation of adaptive cellular stress response signaling pathways that enhance mitochondrial health, DNA repair and autophagy. PF also promotes stem cell-based regeneration as well as long-lasting metabolic effects. Randomized controlled clinical trials of IF versus PF and isoenergetic continuous energy restriction in human subjects will be required to establish the efficacy of IF in improving general health, and preventing and managing major diseases of aging.

The objective of this review is to provide an overview of intermittent fasting regimens, summarize the evidence on the health benefits of intermittent fasting, and discuss physiological mechanisms by which intermittent fasting might lead to improved health outcomes. A MEDLINE search was performed using PubMed and the terms "intermittent fasting," "fasting," "time-restricted feeding," and "food timing." Modified fasting regimens appear to promote weight loss and may improve metabolic health. Several lines of evidence also support the hypothesis that eating patterns that reduce or eliminate nighttime eating and prolong nightly fasting intervals may result in sustained improvements in human health. Intermittent fasting regimens are hypothesized to influence metabolic regulation via effects on (a) circadian biology, (b) the gut microbiome, and (c) modifiable lifestyle behaviors, such as sleep. If proven to be efficacious, these eating regimens offer promising nonpharmacological approaches to improving health at the population level, with multiple public health benefits.

Intermittent fasting (IF) is a dietary protocol where energy restriction is induced by alternate periods of ad libitum feeding and fasting. Prophylactic intermittent fasting has been shown to extend lifespan and attenuate the progress and severity of age-related diseases such as cardiovascular (e.g. stroke and myocardial infarction), neurodegenerative (e.g. Alzheimer's disease and Parkinson's disease) and cancerous diseases in animal models. Stroke is the second leading cause of death, and lifestyle risk factors such as obesity and physical inactivity have been associated with elevated risks of stroke in humans. Recent studies have shown that prophylactic IF may mitigate tissue damage and neurological deficit following ischemic stroke by a mechanism(s) involving suppression of excitotoxicity, oxidative stress, inflammation and cell death pathways in animal stroke models. This review summarizes data supporting the potential hormesis mechanisms of prophylactic IF in animal models, and with a focus on findings from animal studies of prophylactic IF in stroke in our laboratory.

There are more search results also, it seems that research is ongoing to do with fasting. What I added to the other topic about the potential risks of fasting was to do with longer term fasts or low calorie diets, in general (you don't want to get too carried away with it).

Is it not a problem that one will not meet one's daily nutritional needs during the fasting?

It depends, because hyponatremia could be a problem if you were sweating a lot, or drinking too much water, without replacing sodium in the process. Being dehydrated can also damage the kidneys, so drinking too little water could be a problem too. Or too much salt can cause dehydration and salt poisoning ("drinking seawater to maintain hydration is counterproductive; more water must be excreted to eliminate the salt, via urine, than the amount of water obtained from the seawater itself... some people have claimed that up to two cups a day, mixed with fresh water in a 2:3 ratio, produces no ill effect"). I was reading some articles about heat exhaustion, and it's important to maintain that balance, especially if you're working in the heat.

During exercise the ability of the kidney to excrete water is restricted, and therefore, there is a risk of hyperhydration and hyponatremia, mainly under conditions of overdrinking. Sodium loss through sweating and the development of hyponatremia will primarily occur during strenuous exercise lasting more than 4 hours—Fluid and sodium balance during exercise

People working in moderately hot conditions for 10 hrs on average will lose between 4.8 and 6 g of sodium (Na) equivalent to 12–15 g of salt (NaCl) depending on acclimatisation. However due to the substantial interindividual variation in sweat rate and sodium concentration individual losses may be much higher. This essential electrolyte must be replaced in order to avoid fluid imbalances, thus eating during the shift is a must.—Sweat rate and sodium loss during work in the heat

There is now evidence that chronic or repeated episodes of heat stress accompanied by water and solute loss (by dehydration or volume depletion) can cause repeated subclinical ischemic kidney injury, which over time may lead to permanent kidney damage and chronic kidney disease.—Occupational Heat Stress and Kidney Health

The patient reported sweating excessively while performing 90 min of strenuous exertion in a humidified room heated to an ambient temperature of 40.6°C. After the workout she drank 3.5 litres of water before experiencing breathlessness, severe muscle cramps, nausea and general malaise... This case highlights the dangers of overzealous fluid replacement following severe exertion in a hot environment.—Exercise associated hyponatraemia leading to tonic-clonic seizure

It would seem that a low-sodium diet would not be the best counterpart to intermittent fasting, even so far as an average amount of sweating and drinking water goes (you probabaly don't want to follow low-sodium with no sodium, and so on, especially in case you get stuck somewhere on a hot day...)!

Lately I have been wondering if eating too frequently (3 meals + snakes) is problematic in terms of glucose and fatty acid metabolism, in particular when the meals/snacks are always relatively rich in carbohydrates. I've seen some anecdotes of people's glucose control being improved by reduce meal frequency and only snacking, if you must, on low-carbohydrate snacks.

I guess it depends on what your activities are in the process there too. I was reading that children would more likely experience hypoglycemia while fasting 16 hours or more. Generally that's not a problem for the average person though (unless maybe it's around 72 hours or more).

As fasting period is prolonged, glycogen stores are depleted and plasma glucose levels decline. This stimulates the secretion of glucagon, epinephrine, growth hormone, and cortisol, which promote hepatic gluconeogenesis, lipolysis and ketogenesis. If fasting is prolonged 24–48 hours, glucose utilization by fat and muscle decreases significantly; insulin levels are suppressed and lipolysis and ketogenesis increase and ketones become the significant fuel source for the brain.—Glucose Counterregulatory Responses to Hypoglycemia

This may also be referred to as a ketogenic diet, which could involve reducing carbohydrates as well.

Calorie restriction in animals is achieved by either daily reduction of food intake or intermittent fasting. Both protocols induce similar physiological and metabolic changes except for one important difference: intermittent fasting leads to a much larger increase in blood levels of the ketone body β-hydroxybutyrate. Interestingly, this rise in β-hydroxybutyrate concentration is associated with a more significant reduction in the vulnerability of hippocampal neurons to kainate injections. The ketogenic property of fasting was recognized several decades ago and lead to the formulation of the high-fat, low-carbohydrate ketogenic diet in the 1920s. The anticonvulsant properties of the ketogenic diet are currently well recognized, but more recent data suggests that the ketogenic diet is neuroprotective as well and that the underlying mechanisms are similar to those activated by calorie restriction.—The Neuroprotective Properties of the ketogenic diet

By the way, the oral rehydration salts are sold in packets that could be carried around easily, in case you were in the middle of a water fast and could use something else immediately, due to nutrient depletion from sweating or whatever. Basically some salt and sugar packets would cover most of it (and salt substitutes have potassium chloride too, while the citrate or bicarbonate may not be absolutely necessary: Oral rehydration solution without bicarbonate; it's one of those DIY subjects for survivalists too, like wearing an ammo belt with spices on it)!

The authors analyzed 208 sweat samples to determine losses of iodine, sodium, potassium, and calcium in sweat...
The results of the study suggest that loss of iodine through profuse sweating may lead to iodine deficiency, and loss of electrolytes through sweating may have a dietary significance for heat-stressed individuals or for individuals who perform heavy workloads.—Electrolyte loss in sweat and iodine deficiency in a hot environment

Looks like marathoners make this kind of thing too, these days (since some were dropping dead).

Give the body sodium, glucose and water and hydration occurs without causing a dangerous decrease in the sodium in the blood. The glucose also gives you the extra calories (aka. energy) you need, in the form easiest for your body to use. This will also help you prevent feeling like you’ve "hit the wall" during exercise.

The other important player – potassium. Potassium is essential for heart, nerve and kidney function, acid-base balance in the body, glycogen (energy) storage in the muscles and liver... I could go on. It is also depleted by fluid losses from the body. On average, one loses about 150mg potassium in a liter of sweat.

Hyponatremia is said to be the most commonly seen water–electrolyte imbalance. This is the part of balancing a diet that could make intermittent fasting like walking a highwire, and something like ORS would be a safety net.